There is known a method for breaking direct current, whereby at a moment of breaking direct current, an oscillatory discharge is superposed on the direct current. The oscillatory discharge is calculated so that the total current through the circuit should cross zero after a short period of time (cf. A. M. Zalessky, "Osnovy teorii electricheskikh apparatov"/"The Theoretical Principles of Electrical Apparatus"/, Moscow, 1974, p. 123). The device for effecting this method features an extremely complicated circuitry and high power consumption for its operation.
There are further known methods for breaking direct current, whereby direct current is passed through low-pressure gas discharge devices. According to one such method, there is brought about an increase in the negative potential at the grid of the gas discharge device, which, in turn, brings about a growth of the space charge layer in the small holes of the grid and a breaking of the direct current passed through the device. This method is feasible at a high level and high rates of increase in the return voltage, but it can only be used with currents of limited magnitudes because of great power losses at the grids (cf. A. I. Geren, V. A. Krestov, A. A. Nikolayev, "Moshchnye metallokeramicheskiye tasitrony"/"High-Power Metal-Ceramic Tacitrons"/, the Journal Radiotechnika, vol. 28, No. 3, 1973).
According to another method, direct current is passed through a gas discharge device, wherein the discharge is maintained by an external magnetic field which is either totally removed or has its intensity brought down below a critical value (cf. U.S. Pat. No. 3,678,289). In this case the permissible rate of increase in the return voltage is limited by the great period of deionization of the plasma gap, which may be as long as 100 to 1,000 microseconds. As a result, with an increase in the intensity of direct current to be broken and with an increase in the return voltage, the direct current breaking process may be disturbed as a certain discharge turns into an electric arc with a cathode spot.
There is still further known a method for breaking direct current, whereby direct current is passed through a low-pressure gas discharge device. At the same time the density of the gas filling the current channel of the gas discharge device is reduced to a critical value which causes a breaking of the direct current. For this purpose, a barrier is set across the path of the discharge inside the gas discharge device. The barrier is provided with holes to narrow the current channel. The density of gas is reduced in the narrow portion of the current channel by ions and electrons traveling at high speeds, which pass the direct current through the device and force neutral gas atoms to the area which is adjacent to the narrow portion of the current channel.
The plasma decay, which breaks the direct current, occurs only in the narrow portion of the current channel, whereas a state of conduction is maintained for some time in the area adjacent to the narrow portion of the current channel, so right after the breaking of the direct current, the conducting area may close and the discharge may turn into sustained current oscillation. Such a situation is all the more probable if direct current is broken under conditions of switching overvoltages. In addition, with an increase in the intensity of direct current to be broken, when the compression by the proper magnetic fields makes the diameter of the discharge channel less than that of its narrow portion, the method under review cannot be effected at all because the current channel moves at a high speed over the narrow portion and thus compensates for the decrease in the density of gas, caused by the passing current. As a result, the method under review is only effective with limited current and voltage levels; the permissible rate of increase in the voltage is limited by the great period of deionization of plasma in the area adjacent to the narrow portion of the current channel.
It is possible to break direct current in any known low-pressure gas discharge device if one can reduce in some way or other the density of gas inside the gas discharge device to a certain critical value which is dependent upon the nature of the gas and the configuration of the electrodes in the discharge zone of the device. However, such devices are only applicable to currents and voltages of limited magnitudes. Besides, the operating speed of such devices is limited because the decrease in the density of gas is accompanied by either a sharp increase in the voltage drop or by a rapid failure of the hot cathode, or by a growing probability of the formation of a cascade arc, or other undesired phenomena. For these reasons, users of such devices avoid great changes in the gas density and try to stabilize the gas conditions with the aid of a gas generator; in addition, the electrodes are manufactured from a material having a limited sorptive capacity with regard to the gas filling the gas discharge device.
There is known a switching means which can be used for breaking direct current and which is built around a gas discharge device. In said gas discharge device, the cathode is formed by an end emitter, a reflector arranged opposite the surface of the emitter, and two coaxial cylinders. Intermediate electrodes are interposed between the cathode and anode.
The cathode of the above design is such that a considerable amount of current passing through the device is shorted through the cold electrodes, whereas the discharge current is chiefly carried by ions formed due to ionization of neutral atoms by fast electrons oscillating between the surface of the hot cathode's emitter, the reflector and the coaxial cylinders.
In such a device, the cathode voltage drop is not varied by more than 20 percent with a more than 100-fold decrease in the density of gas; the passage of current is accompanied by an intensive absorption of gas by the reflector and the coaxial cylinders encompassing the hot cathode.
The device under review can be used for breaking direct current through decreasing the density of gas to a critical value; however, the magnitudes of currents to be broken are limited because the amount of current passing through the reflector increases and may amount to more than 80 percent of the total discharge current. In addition, a sharp rise in the temperature of the reflector and a reduced amount of current passing through the coaxial cylinders limit the rate of absorption of gas by the electrodes, wherefore it takes too much time or is totally impossible to reach a critical gas density.